Composite fuse links of silver and copper



Oct. 8, 1957 K. w. swAlN COMPOSITE FUSE LINKS 0E slm/ER AND COPPER FiledOct. 4, 1954 nite r1 Kenneth W. Swain, Hampton Falls, N. H., assigner toThe Chasehawmut Company, Newburyport,

Application October 4, 1954, Serial No. 460,177

13 Claims. (Cl. 20D- 435) This invention relates to the protection ofelectric circuits, electric apparatus and electric machinery by electricfuses.

It is one object of this invention to take advantage of the differenceof the properties of silver and copper, respectively, to produce fuselinks for current-limiting fuses capable of etecting rapid initiation ofthe interrupting process, i. e. rapid arc initiation, at the occurrenceof major fault currents, which fuse links have a relatively high heatabsorbing capacity and heat dissipating ability conducive to time lag inthe small overload range, and conducive to a relatively highcurrent-carrying ability.

Another object of the invention is to provide fuse links for so-calledsilver-sand fuses wherein a portion of the link silver heretofore neededin so-called silver-sand fuses of comparable rating is dispensed with,and replaced by, an amount of copper tending to increase the currentrating of the fuse.

Another object of the invention is to provide composite fuse links ofsilver and copper having an uninterrupted current path of silver fromone end or terminal of the link, to the other end or terminal thereof.

Still another object of the invention is to provide composite fuse linksof silver and copper which lend themselves readily to the formation ormultibreaks in series, and thus to producing high arc voltages, and inwhich fuse links each of the breaks is formed at a reducedcrosssectional area consisting wholly of silver.

Further objects and advantages of the invention will become moreapparent as the following description proceeds, and the features ofnovelty characterising the invention will be pointed out withparticularity in the claims annexed to, and forming part of, thisspecification.

For a better understanding of the invention reference may be had to theaccompanying drawing in which Fig. l is a side elevation of the materialused for manufacturing fuse links embodying my invention;

Fig. 2 is a section along 2 2 of Fig. l;

Fig. 3 is substantially a longitudinal section through a fuse includinga fuse link embodying my invention;

Fig. 4 is a transverse section along 4-4 of Fig. 3

Fig. 5 is in substance a longitudinal section through a fuse including afusing link embodying my invention and designed to produce multibreaksresulting in relatively high arc voltages; and

Fig. 6 is a section along 6 6 of Fig. 5.

The material shown in Figs. 1 and 2 comprises a strip of silver 2 andtwo spaced strips of copper 1 each arranged one side of the strip ofsilver 2. Strips 2 and 1 are bonded together to form a substantiallyunitary strip of sheet metal. The process for producing such compositemetals are well known in the art, and therefore do not need to bedescribed here in detail. The aggregate crosssectional area of thestrips of copper 1 is larger than the cross-sectional area of the stripof silver 2. This tends to minimize the` cost of the composite strip perunit of current carrying ability thereof, and has additional advantagesas will be readily apparent from the ensuing description of Figs. 3 6.

States Patent() f' 2,809,257 Patented Oct. 8, 1957 ICC Referring now toFigs. 3 and 4, the fuse shown therein comprises a fuse link made of acomposite metal of the kind shown in Figs. l and 2. Strips 1 of copper`and strip 2 of silver bonded together along longitudinal edges thereofextend in a direction longitudinally of the fuse and conductivelyinterconnect the two terminal caps or ferrules 5. The two ferrules S aremounted on a tubular casing of an appropriate insulating material, e. g.a glasscloth-synthetic-resin laminate. Each of the ferrules 5 isprovided with a slot for insertion of one terminal of the fuse link 1,2. The terminals or axially outer ends of fuse link 1 2 may be slightlyreduced in width compared to what may be called the overall width of thelink 1, 2. Such a reduction in width may be desirable for manufacturingreasons but has no effect upon the performance characteristics of thefuse since the size reduction is effected at au inactive point of thefuse link. Each slot in each of the caps or ferrules 5 is substantiallyclosed by a washer S to preclude the silicious arc-quenching filler 7within casing 3 to iiow through the slots in fcrrules S out of thecasing 3. Each ferrule S is soldered, as generally done in the art, toone of the axially outer ends of terminals of fuse link 1, 2 to minimizethe resistance of the current-path through the fuse. An axially outercap or ferrule 6 is mounted under pressure on each of the axially innercaps or ferrules 5, and both the outer and the inner caps or ferrulesare crimped at their axially inner ends to produce a firm and gas-tightconnection with casing 3. Each of the outer caps or ferrules 6 isprovided with a blade Contact 4 extending away from the casing 3 of thefuse. Fuse link 1, 2 is provided with a neck 2o, or reducedcross-section portion, the cross-sectional area of which determines thefault current which must fiow through the fuse to cause the neck to fuseand to vaporize, and thus to initiate the interrupting process, or toinitiate arcing within the fuse. Neck 2a is formed by punching twojuxtaposed portions out of the link, and thus making two juxtaposedincisions into the link. The transverse width of these incisions exceedsthe width of each of the two copper strips resulting in a local completeinterruption of the current-path through the copper strips 1, and alocal reduction of the cross-sectional area of the current-path throughthe strip of silver 2, The specific heat of silver is `05S and thespecific heat of copper is .092. The specific heat of a metal isindicative ot the amount of energy required to cause fusion of aconductor having a predetermined geometry, and under comparableconditions silver is the metal which requires a minimum of fusingenergy. The neck 2a consisting wholly of silver is therefore conduciveto limiting the peak of the letthrough current which may flow throughthe fuse on occurrence of a short-circuit current, or an equivalentmajor fault current. The outer strips 1 of copper are effective heatabso-rbers and heat dissipators. lt will be apparent from the aboveiigure relative to the specific heat of copper that the heat absorbingand cooling action of the two strips, o1' fins, of copper is moreeffective than that of the lateral portions of a comparable ribbon typefuse link made entirely of silver, and consequently considerably moreexpensive than the fuse link 1, 2 shown in Fig. 2.

lt is important that all copper be removed from the region of arcinitiation to limit as much as possible the amount of heat required tocause fusion and vaporization of the narrowest point of the link 1, 2.

Referring now to Figs. 5 and 6, the fuse link provided in the structureshown therein comprises a pair of spaced strips 16 of silver and anintermediate, substantially coextensive strip 9 of copper bonded at thelongitudinal edges thereof to said pair of strips of silver to form avirtually unitary strip of sheet metal. The pair of strips 10 of silverhave one or more pairs of juxtaposed incisions 10b,

each reducing the cross-sectional area of one of the pair of strips it)of silver without interrupting the continuity thereof. The copper strip9 is provided with one or a plurality of punched out `portions 9a, eachsubstantially in registry with one of said pair of juxtaposed incision10b. The punched out portion or portions 9a are wider thanand completelysever the copper strip 9 into parts solely held together by saidv pairof strips it? of silver. In addition thereto, the punched out portion orportions 9a effect a further reduction of the Cross-sectional area ofthe strips it) o silver, i. e. a reduction of the cross-sectionai areaof strips ld beyond that eifected by juxtaposed incisions 10b. As aresult of incisions 10b in silver strips l@ and of the punched outportions or perforations 9b in copper strip which extend into silverstrips l@ a plurality of pairs of necks, or reduced cross-sectionportions ida are formed in silver strips l0. Each of these pairs ofnecks or reduced cross-section portions i051 in silver strips 1t? isadapted to form a pair of parallel current paths. Since these currentpaths are zones of minimum cross-sectional area, initial fusion and arcinitiation will occur at these particular' points on the occurrence ofshort-'circuit currents, or iike major fault currents. Each pair ofnecks 10a will result in the formation of two arclets or breaks inparallel, and serially related pairs of necks 10a will result in theformation of serially related pairs of arclets or breaks. Since eachneck ida consists wholly of silver the instantaneous current requiredfor initiating the interrupting process tends to be very low, yet thecurrent `carrying capacity of the link tends to be high on account ofthe heat dissipating action of the portions or section of copl ersituated to both sides of each perforation 9a.

Fuse link 10, 9, It@ is conductively connected to terminal blocks 1lwhich are disc shaped and provided with radial grooves i2, foraccommodating the axially outer ends of link 10, 9, 10. Each terminalblock 11 is provided with a blade contact le forming an integral partthereof, and the tubular casing 13 of insulating material is mounted on,and supported by, terminal blocks 11. Transverse pins l of steelestablish a firm mechanical connection between casing 1S and terminalblocks ll. The inside of casing 13 is iilled with a pulverulent arcextinguishing iiller lo, preferably clean quartz sand, i. e. quartz sandfrom which iron and other impurities have been carefully removed.

To increase as much as possible the saving of silver which this designenables to achieve, the aggregate crosssectional area of the pair ofspaced strips of silver ought to be relatively small compared to thecross-sectional area of the intermediate substantially coextensive strip9 of copper. As applied in this context the term cross-sectional arearefers to the points of the fuse link where the silver strips 1t)- `andthe copper strip 9 have their normal width rather than to the points ofthe link where the cross-sectional area of the silver strips 10 has beenreduced and Where the 4copper strip 9 has been severed.

It will be apparent from the foregoing that the incisions 10b and theperforations 9a which may both be produced by simultaneously performedpunching operations subdivide link l0, 9, i@ into a plurality ofserially related current paths of relatively large crosssectional areaformed jointly by copper and silver alternating with current paths ofrelatively small cross-sectional area formed by silver only. At thepoints where the current path is formed by silver :as well as copper thecurrent flows preponderantly in silver sections rather than in coppersections. This is mainly due to the geometrical configuration of thelink causing the copper sections to operate mainly as heat dissipators,rather than as conductors of current.

The transverse punched out portions 9a do not necessarily need to besquare in shape. This shape has been found to be particularly desirable,but the punched out portions 9a may be even circular, if desired.

In some instances it may be desirable to add some Cil i derating meansto the fuse link to increase the time lagr thereof. Such a deratingmeans has been shown in Fig. 3 in the form of a tin rivet 2b insertedinto the strip of silver 2 immediately adjacent to neck Za.

The term silver as used in this context is intended to include alloys ofsilver which have substantially the same low melting energy as silver,and 'the term copper as being used in this context is intended toinclude alloys of copper which have thermal properties comparable tothose of copper.

it will be understood that i have illustrated and described hereinpreferred embodiments of the invention and that various alterations maybe made in the details thereof without departing from the spirit or thescope of the invention as defined in the appended claims.

I claim:

l. A composite ribbon-type fuse link for current-limiting fusescomprising at least one strip of copper and at least one strip of silverbonded together along the longitudinal edges thereof to form a virtuallyunitary strip of sheet metal, said strip of copper projecting laterallyfrom at least one of the longitudinal edges of said strip of silver tooperate as a lateral cooling iin for said strip of silver, and saidstrip of copper and said strip of silver having at least one punched outportion of larger width than the width of said strip of copper resultingin a local interruption of the current-path through said strip of copperand a local reduction of tl e cross-sectional area of the current-paththrough said strip of silver.

2. A composite ribbon-type fuse link for current-limiting fusescomprising at least one strip of copper and at least one strip of silverbonded together along the longitudinal edges thereof to form a.substantially unitary strip of sheet metal, said copper strip and saidsilver strip being generally coextensive from one terminal of said linkto the other terminal thereof, said copper strip and said silver striphaving a punched out portion at least at one point thereof situatedbetween the terminals of said link, and the width of said punched outportion exceeding the width of said copper strip.

3. A composite ribbon-type fuse link for current-limiting fusescomprising strips of copper and of silver, including a pair of parallelstrips of the same meta1 and a strip of a different metal arrangedbetween and bonded to said pair of strips of the same metal to form avirtually unitary strip of sheet metal, said pair of strips of the samemetal projecting laterally from the longitudinal edges of said strip ofa different metal, and said link having at least one punched out portionsituated between the ends thereof larger in width than the width of the`constituent copper of said link at the point thereof where said portionis situated to cause a local interruption of the current-path throughcopper and a local reduction of the cross-sectional area of thecurrent-path through silver.

4. A composite ribbon-type fuse link for current-limiting fusescomprising substantially coextensive strips of copper andof silverbonded together at the longitudinal edges thereof to form a virtuallyunitary strip of sheet metal, the cross-sectional area of copper in saidlink generally exceeding the ycross-sectional area of silver therein,and said link having between the ends thereof at least one pair oflateral incisions each larger in width than the width of each of saidstrips of copper.

5. A composite ribbon-type fuse link for current-limiting fusescomprising a pair of substantially coextensive spaced strips of copperand an intermediate strip of silver bonded together at the longitudinaledges thereof to form a virtually unitary strip of sheet metal, thecross-sectional area of copper in said link generally exceeding thecross sectional area of silver therein, and said link havingsubstantially midway between the ends thereof one point of generallyreduced cross-sectional area formed by a pair of perforations each oflarger width than the width of each of saidpair of strips of copper.

6. A composite ribbon-type fuse link for current-limiting fusescomprising strips of copper and of silver including a pair of parallelspaced strips of the same metal extending in a direction longitudinallyof said link and a strip of a different metal arranged between andbonded to said pair of strips of the same metal at the longitudinaledges thereof to form a virtually unitary strip of sheet metal, and saidlink having a substantially transverse punched out portion substantiallymidway between the ends thereof, the width of said punched out portionbein-g less than the total width of said link but exceeding the totalwidth of said strips of copper.

7. A composite fuse link for current-limiting fuses comprising a pair ofspaced strips of copper and an intermediate generally coextensive stripof silver extending in a direction longitudinally of said link andbonded at the longitudinal edges thereof to said pair of strips ofcopper to form a virtually unitary strip of sheet metal, said linkhaving at least one pair of juxtaposed substantially V- shaped incisionseach of greater depth than the width of one of said pair of copperstrips and each completely severing one of said pair of copper strips ata predetermined point between the ends thereof, and said pair ofincisions reducing the cross-sectional area of said silver strip withoutinterrupting the current-path therethrough.

8. A composite fuse link for current-limiting fuses comprising a pair ofspaced strips of silver having a relatively small aggregatecross-sectional area and an intermediate substantially coextensive stripof copper having a relatively large cross-sectional area bonded at thelongitudinal edges thereof to said pair of strips of silver to form avirtually unitary strip of sheet metal, said link having at least onepair of neck portions wholly consisting of silver adapted to lie inparallel in an electric circuit into which said link is inserted.

9. A composite ribbon-type fuse link for current-lirniting fusescomprising at least one strip of copper and at least one strip of silverbonded together at the longitudinal edges thereof to form a virtuallyunitary strip of sheet metal, said link having between the ends thereofat least one substantially transverse incision of larger width than saidstrip of copper totally severing said one strip of copper.

10. A composite ribbon-type fuse link for currentlimiting fusescomprising at least one strip of copper and at least one strip of silverbonded together at the longitudinal edges thereof to form a virtualyunitary strip of sheet metal, said link having a plurality of seriallyarranged punchings subdividing said link into a plurality of seriallyrelated current-paths of relatively large crosssectional area formedjointly by copper and silver alternating with current-paths ofrelatively small cross-sectional area formed by silver alone.

11. A composite ribbon-type fuse link for currentlimiting fusescomprising at least one strip of copper and at least one strip of silverbonded together at the longitudinal edges thereof to form a virtuallyunitary strip of sheet metal, the cross-sectional area of copper in saidvirtually unitary strip being large compared to the cross-sectional areaof silver therein, and a plurality of punched out portions spaced in adirection longitudinally of said virtually unitary strip, the width ofeach of said punched out portions exceeding the width of said strip ofcopper and each of said punched out portions constituting a completebreak of the current-path througth copper and a reduction of thecross-sectional area of the current-path through silver.

l2. A composite ribbon-type fuse link for current-limiting fusescomprising a pair of spaced strips of silver and a strip of copperarranged between and bonded at the longitudinal edges thereof to saidpair of strips of silver to form a virtually unitary strip of sheetmetal, the crosssectional area of said strip of copper exceeding theaggregate cross-sectional areas of said pair of strips of silver, and aplurality of spaced punched out portions in said virtually unitary stripextending substantially in transversal direction thereof, each of saidpunched out portions exceeding in width the width of said strip ofcopper and each of said punched out portions constituting a completelocal break of the current path through copper and a local reduction ofthe cross-sectional area of the current-path through silver.

13. A composite ribbon-type fuse link for currentlimiting fusescomprising a pair of spaced strips of silver and a strip of copperarranged between and bonded at the longitudinal edges thereof to saidpair of strips of silver to form a virtually unitary strip of sheetmetal, the crosssectional area of said strip of c'opper exceeding theaggregate cross-sectional areas of said pair of strips of silver, apunched out portion in said virtually unitary strip extendingsubstantially in transverse direction thereof and being of larger widththan the width of said strip of copper to locally completely break thecurrent-path through `Said strip of copper and to locally reduce thecrosssectional area of the current-path through each of said pair ofstrips of silver, and a pair of lateral incisions in each of said pairof strips of silver locally reducing the cross-sectional area of thecurrent-path through each of said pair of strips of silver to a largerextent than effected by said punched out portion.

References Cited in the file of this patent UNITED STATES PATENTS1,856,317 Clark May 3, 1932 2,703,352 Kozacka Mar. 1, 1955 FOREIGNPATENTS 349,519 Great Britain May 27, 1931 586,586 Great Britain Mar.24, 1947

